[R] extracting submatrix from a bigger one

[William Dunlap]

> First of all, thanks for all the replies!!
> What you have written helps, but is not entirely the answer to my problem.
> 
> What I'd have is the creation of new data.frames each of one named with
> the ID of the original dataframe and with all the columns.

What was suggested gave you a list of data.frames, each named with the ID .
You can use the syntax list$name or list[["name"]] to refer to a data.frame.
  R> splitData <- split(allData, allData$ID)
  R> splitData$x1
    ID value1 value2
  1 x1     10     12
  2 x1     12     22
  3 x1     11      9
  R> splitData$x2
    ID value1 value2
  4 x2     15     10

You seem to want a function that creates a bunch of data.frames in the
current environment instead of one that creates them in a list created to
hold them.  This is not necessary and actually gets in the way most of the
time.

If you want to refer to 'x1' instead of 'splitData$x1' you can use 'with', as in
  R> with(splitData, mean(x1$value2) - mean(x2$value2))
  [1] 4.333333
instead of the slightly wordier
  R> mean(splitData$x1$value2) - mean(splitData$x2$value2)
  [1] 4.333333

If you want to process each sub-data.frame (these are data.frame, not matrices)
you can use lapply() or sapply() or vapply() on the list
  R> dm <- sapply(splitData, function(x)mean(x$value2) - mean(x$value1))
  R> dm
         x1        x2        x3 
   3.333333 -5.000000 -2.500000
  R> dm["x2"]
  x2 
  -5

If you put all those names into the current environment you stand the chance
of clobbering some other dataset whose name matched one of the entries in
allData$ID.  Also you would have to use some rather ugly code involving get()
and assign() to manipulate the objects.  Learn to love lists.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of matteo
> Sent: Monday, June 24, 2013 11:32 AM
> To: Bert Gunter
> Cc: r-help at r-project.org
> Subject: Re: [R] extracting submatrix from a bigger one
> 
> First of all, thanks for all the replies!!
> What you have written helps, but is not entirely the answer to my problem.
> 
> What I'd have is the creation of new data.frames each of one named with
> the ID of the original dataframe and with all the columns.
> 
> For example, in the original dataframe one column (ID) has 5 different
> elements:
> 
> ID    value1    value2
> x1        10            12
> x1        12            22
> x1        11            9
> x2        15            10
> x3        11            11
> x3        13            8
> 
> I need a command ables to split the dataframe in other smallest and
> separated dataframes, so that they look like
> 
> x1 is
> ID    value1    value2
> x1        10            12
> x1        12            22
> x1        11            9
> 
> x2 is
> ID    value1    value2
> x2        15            10
> 
> and x3 is
> ID    value1    value2
> x1        10            12
> x3        11            11
> x3        13            8
> 
> 
> Sorry if I'm not able to explain it better and as I said I'm very new to
> R.....
> 
> Thanks
> 
> Matteo
> 
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